MICROBIAL STATEGIES IN RELATION TO THE HOST'S (CONSTITUTIVE) AND SPECIFIC (INDUCED) DEFENSES

Reading Assignments: (1) Text: Chapter 8 (2) HANDOUTS ABOUT ANTIGENIC VARIATION: (A) Kuby, J. 1997. Immunology, pp. 476-478., W. H. Freeman and Co., N.Y. (B) Volk, W.W., B.M. Gebhardt, M-L Hammarskjold, and R.J. Kadner. 1996. Essentials of Medical Microbiology (5^th ed), pp. 446-447. Lippincott-Raven, Philadelphia, PA.

I. INTRODUCTION

II. STRATEGIES ADOPTED BY PATHOGENS TO THRWART THE NON SPECIFIC (CONSTITUTIVE) DEFENSES:

A. Successful pathogens can REACH THE MUCOSAL SURFACE and OVERCOME THE SURFACE DEFENSES of the skin and mucous membranes (review Lectures 4-6)

B. After invasion across the mucosal surfaces, successful pathogens can subvert PHAGOCYTOSIS and SURVIVE IN VIVO (review Lecture 7)

1. Strategies of extracellular pathogens

2. Strategies of intracellular pathogens

C. Successful pathogens can defend themselves against COMPLEMENT (See text p. 108 and Fig. 8.1):

1. Pathogens Can Prevent Complement Activation by:

a. coating their surfaces so the complement components needed for activation cannot bind:

i. Bacterial surface is coated with a capsule, preventing the binding of C3b needed for activation of the alternative pathway. Ex.s - capsules of S. pneumoniae (pneumococcus), and N. meningiditis (meningococcus).

ii. Bacterial surface is coated with IgA antibodies, which do not activate complement via the classical pathway. (This blocks the binding of IgM or IgG antibodies which can activate the cascade and/or opsonize the pathogen. (N. meningitidis)

b. appropriating an inhibitor of complement activation into their surfaces:

i. Bacteria incorporate sialic acid into their capsular polysaccharide or add sialic acid to the terminal sugars of their LPS (the lipooligosaccharide of gonococci).

ii. Bacteria or viruses coat their surfaces with a protein that mimics a complement control protein. Vaccinia viruses encode a “C4bp-like protein” which binds to the C4b protein, limiting complement activation by the classical pathway and accelerating the decay of the classical C3 convertase. (Review the regulation of the complement pathway.)

2. Pathogens can have Surfaces that Prevent Insertion of the membrane attack complex “ The MAC”:

a. A built in structural barrier – the thick peptidoglycan layer of the Gram + cell wall.

b. The MAC cannot reach the Gram - outer membrane to insert. Long chain (“smooth” LPS) of G- enterics (E.coli and Salmonella) prevent the MAC from reaching the outer membrane and inserting.

3. Pathogens Can Produce enzymes that destroy complement “split products.

a. Enzymes produced by P. aeruginosa (elastase) inactivate C3a and C5a; Group A streptococci secrete C3b peptidase that cleaves C3b.

D. Some Viruses Can Defeat INTERFERON

1. Some viruses are poor inducers of interferon.

2. Some viruses can interfere with the antiviral proteins which are induced by interferon.

III. STRATEGIES ADOPTED BY PATHOGENS TO THWART THE SPECIFIC (INDUCED) DEFENSES:

A spy in a foreign country has many strategies to avoid being caught. He/she can conceal his presence from the police by hiding or by adopting the disguise of a native. Pathogens have some of the same choices.

A. CONCEALMENT- MICROBES COLONIZE PRIVILEGED SITES OUT OF THE REACH OF THE CIRCULATING LYMPHOCYTES, or DISGUISE THEMSELVES SO THEY ARE NOT RECOGNIZED BY THE IMMUNE SYSTEM. ON THE SKIN AND MUCOUS MEMBRANES:

1. The Normal Flora

Microbes of the normal flora colonize the skin and body's mucosal surfaces, without eliciting a significant inflammatory response. They are shed from these surfaces directly into the external secretions – living their lives for the most part, out of the reach of the circulating lymphocytes. Some pathogens that employ this same strategy:

2. Human papilloma viruses and Warts

3. The “Hit and Run” Viral Pathogens of the Mucosal Surfaces

4. Microbial Infections of Secretory or Excretory Glands

Viruses and bacteria that infect the epithelial surfaces of the

secretory or excretory glands such as the mammary

gland, salivary gland, kidney tubule or bile duct also are out of

reach of the circulating lymphocytes.

In (3) and (4) above, secretory IgA is made and can bind to pathogens at these sites neutralizing them; however, sIgA is unable to kill pathogens and usually does not totally control the replication of microbes at these sites.

WITHIN THE BODY

Within the body, it is more difficult to avoid lymphocytes and antibodies, but certain sites are safer than others.

(5) Pathogens are Safer in Privileged Tissues Within the Body:

the central nervous system, joints, testes, and placenta.

Lymphocyte circulation is less intense in these sites, and there is more restricted access of antibodies and complement. However, if an inflammatory response is induced - lymphocytes, monocytes and antibodies are rapidly delivered and the site is no longer “privileged”.

(6) Pathogens Can Hide Latently Inside of Cells

If a pathogen could remain inside cells without allowing its

antigens to be displayed on the cell surface, it would be incognito as far as the immune defenses are concerned. However, the host has “informers” – the MHC which pick up and transport endogenously processed peptides from intracellular pathogens and displays them on the cell surface for the immune system to recognize and respond to.

Only viral pathogens that establish latent infections (where the virus genome is present but infectious viruses are not produced) can avoid this fate. In this situation, viral genes are repressed, viral gene products are not expressed and therefore are not displayed for recognition by the immune response. The virus enjoys total anonymity. Ex.'s HIV (proviral DNA integrated into the host genome) and herpesviruses (genome carried for life in spinal root ganglia).

.

(7) Pathogens can have cell surface antigens that mimic (or cross

react) with host antigens.

Molecular mimicry: if a pathogen surface contains antigens that closely resemble the host, it might not be recognized by the immune system.

Numerous pathogens share antigens that mimic host tissue. Ex. Antigens on the surface of Group A strep are closely related to human heart muscle. However, the host still makes an immune response because the antigens are not identical. This works to the detriment of the host because the antibodies made cross react with cardiac muscle causing immune damage to the heart – rheumatic heart disease.

(8) Pathogens can cloak themselves in upside down antibodies.

Used by a variety of viruses and bacteria. Microbes encode for Fc receptors, which are displayed on their surfaces and bind immunoglobulin molecules in a useless upside down position. This prevents the access of specific antibodies that might opsonize that pathogen or activate the complement cascade via the classical pathway.

(9) Pathogens create their own privileged sites in the body.

S. aureus can deposit fibrin around itself in an attempt to wall itself off from the phagocytic cells. Extreme example: formation of the hydatid cyst that develops in liver, lung or brain around growing colonies of the tapeworm Echinococcus granulosus. Inside the cysts, the worms can survive even though the blood of the host contains protective levels of antibody.

B. ANTIGENIC VARIATION (The spy repeatedly changes his appearance to confuse the police)

1. Used by a wide range of viruses, bacteria, and protozoa. Antigenic variation can occur:

a. during the course of an infection in an individual. The pathogen change its antigens during an infection, enabling it to undergo renewed growth (e.g. trypanosomiasis or relapsing fever).

b. during the spread of the microbe through a population. The pathogen changes its antigens, enabling it to come back and reinfect the same individual (e.g. influenza).

2. Three mechanisms for antigenic variation at the molecular level:

a. Mutation (e.g. antigenic drift in Influenza viruses and other rapidly mutating RNA viruses)

b. Reassortment of viral genes (e.g. antigenic shift in Influenza viruses)

c. Gene switching (e.g. African Trypanosomes, Borrelia recurrentis, and Neisseria gonorrhoeae)

i. African Trypanosomes- African Sleeping Sickness

Gene switching - first demonstrated in African Trypanosomes - African Sleeping Sickness - a chronic debilitating disease transmitted to human by the bite of the tsetse fly. Trypanosomes are inoculated into the bloodstream, multiply in the blood and progress to the CNS - causing menigoencephalitis and the loss of consciousness (“sleeping sickness”).

10% of the trypanosome genome consists of genes for the surface coat of the pathogen – the variant surface glycoprotein (VSG) which covers the entire surface of the organism. (There are genes for approx. 1000 different VSG's.)

The host is bitten by the tsetse fly and the infection is initiated; the host mounts a humoral immune response. The protozoan changes coats, and the immune system responds again. The trypanosomes are able to persist while the immune system is constantly trying to catch up. The patient experiences waves of infections at regular intervals (1 week to 10 days apart).

On the molecular level, each trypanosomes carries more than 1000 genes, each encoding a different VSG primary sequence. The trypanosome expresses only a single VSG gene at a time. Activation of a VSG gene results in duplication of the gene and its transposition to a transcriptionally active expression site at the end of a specific chromosome. Activation of the new gene displaces the previous gene from the expression site. Unknown control mechanisms limit expression to a single VSG expression site at a time.

ii. Borrelia recurrentis – Relapsing Fever

Another vector borne disease – transmitted from human to human by the body louse. (Other species of Borrelia are transmitted by ticks, but louse borne disease occurs in epidemics, especially during wars (with a 50% mortality rate). Estimated 50,000 deaths from relapsing fever in N. Africa during WW II; prevalent today in Egypt and Sudan and in other parts of S. America.

The infection is acquired by the bite of the body louse and the organisms enter the blood and cause multiple lesions in the liver, spleen, kidneys, and G.I tract. After a fever of 4-5 days, the microbes seem to disappear and the patient becomes afebrile for 1 week to 10 days. Then in most cases, a relapse occurs, and the organisms can be recovered from the blood and the internal organs again. After 4-5 days they disappear, but in many cases, the patient relapses 3-10 more times before recovery.

The mechanism - gene switching - similar to that of the trypanosomes. In this case, the Borrelia are switching a major outer membrane protein called the Variable Major Protein. Different VMP's are encoded in linear plasmids and each VMP exists as a separate gene. Only one of these genes is expressed at a time, resulting from transposition of a silent VMP gene to an expression site.

C. INTERFERENCE WITH THE DEVELOPMENT OF THE IMMUNE RESPONSE

1. TOLERANCE- the immune response is not induced or is poorly induced (a lack of responsiveness on the part of the immune system)

2 examples:

a. Occurs during embryonic life, before the immune system is fully developed. The fetus can produce an IgM response but cell-mediated responses are seriously impaired. Ex.s

1. Children with congenital CMV or rubella fail to develop cell mediated responses and take years to clear the virus from the body.

2. Neonatal infection with hepatitis B virus frequently results in permanent carriage of the virus – unknown mechanism.

b. Large quantities of microbial antigens or antigen-antibody complexes circulating in the body can induce tolerance to those antigens. Ex. Cryptococcosis – a disseminated fungal infection caused by the yeast C. neoformans which sheds large amounts of capsular material into the bloodstream.

2. IMMUNOSUPPRESSION

a. Many virus infections cause a general temporary immunosuppression. The host often shows a depressed immune response to antigens of the infecting microbe and to other unrelated microbes.

b. Immunosuppression by microbes may be caused by infection of the immune cells:

i. T cells – HIV, measles

ii. B cells – EBV

iii. Macrophages (HIV, Leshmania)

iv. Dendritic Cells – HIV

The result may be impaired cell function, blockage of cytokine release, or cell death.

c. Microbial toxins and viruses that act as mitogens cause immunosuppression. Bacterial exotoxins (produced by staph and strep) are potent T cell mitogens. They are polyclonal activators of T cells – binding nonspecifically to the Class II MHC molecules on the APC and the TCR - activating T cells nonspecifically; excessive cytokines are released resulting in shock and death. Certain viruses (EBV and HIV) are polyclonal activators of B cells. This causes immunosuppression because the immune cells have been diverted uselessly, and are unable to respond to other potential pathogens.

D. INTERFERENCE WITH THE EXPRESSION OF THE IMMUNE RESPONSE

1. Secretory IgA protease (N. gonorrhoeae, S. pneumoniae and H. influenzae)

2. Expression of Fc receptors by the microbe (protein A by S. aureus; some viruses HSV, VZV, and CMV code for molecules that act as Fc receptors for IgG).